All-optical atomic clock based on coherent population trapping in ⁸⁵Rb

An all-optical microwave frequency standard based on coherent population trapping (CPT) in ⁸⁵Rb is developed. The CPT resonances are detected by an ordinary edge-emitting diode laser in a simple optical setup. A buffer-gas mixture is carefully optimized to yield a narrow linewidth and a reduced temperature dependence of the resonance frequency. With the developed system we are able to measure ultranarrow optically induced hyperfine CPT resonances at <20 Hz, which is in good agreement with the linewidth calculated from experimental parameters. The frequency of an RF-signal generator has been stabilized to the CPT resonance between the two m=0mF=0 magnetic sublevels. The relative frequency stability (square root of Allan variance) follows a slope of 3.5×10⁻¹¹ τ^−1/2(1 s<τ<2000 s). The best stability of 6.4×10⁻¹³ is reached at an integration time of τ=2000 s. This stability is sufficient for many high-precision applications. Frequency-shift measurements were made to evaluate the frequency dependencies on the operation parameters.